Filter devices for filtering a fluid, in particular the air flowing into the passenger compartment of a vehicle, are known in a great number of designs. Generally, the filter device includes a filter element provided with a filter material limited by a border with which a filter element frame is connected. The filter element frame extends at least partly along the border of the filter material. Feasibly, the filter material (in particular a filter paper or a nonwoven filter material) is pleated in zigzag shape so as to increase the filter surface exposed to the fluid flow. In most cases, the filter element frame is disposed extending around the periphery of the filter material; however, with a filter material pleated in zigzag shape it sometimes suffices to make the filter frame extend only along the pleated lateral edges of the filter material. For example, such filter elements are inserted into the fluid inlet opening of a device through which filtered fluid will flow subsequently. In order to prevent fluid flow between the filter element and the edge delimiting the inlet opening of the device, a sealing profile is applied in this area, the profile being a strip generally made of an elastic material, e.g., rubber or foamed material. With a view to the manufacture of the filter element, it is feasible to provide this sealing profile integrally with the filter element.
From German Patent 34 39 255, a filter element is known that has a pleated filter material surrounded by a peripherally extending filter element frame of cardboard material. This filter element frame comprises protruding tongues bent towards the outer side of the filter element frame facing away from the filter material. These cardboard tongues projecting under an acute angle serve as sealing lips resiliently abutting, from inside, the edge of an inlet opening receiving the filter element.
From German Patent 24 29 474, a filter element is known wherein the filter element frame surrounding the filter material is made of a plastics material and has a sealing lip integrally formed thereto which comprises a bending section adjoining the filter element frame and an end portion adjoining the bending section. With a filter element inserted into a filter housing or the like device, the sealing lip is bent in the region of its bending section, whereas it abuts the filter housing in the region of its end portion.
European Patent 0 620 133, European Patent 0 639 474 and Japanese Patent 59-85472 disclose filter elements, wherein the filter element frame enclosing the filter element is provided with an added separate elastic sealing strip.
Finally, European Patent 0 380 026 describes a filter element, wherein a flat filter in the shape of a filter material web is inserted laterally into slots provided in a housing receiving the filter element. The sealing between the filter element and the receiving housing is achieved in particular by the labyrinth-like design of the area between the edges of the filter element and the slots in the receiving housing, which causes a multiple deflection of the fluid flow, thereby resulting in a sealing effect.
For reasons of cost, manufacturers are intent on providing the filter element as a plastic part. Most of the plastic materials used today that can be produced and processed at low cost, however, have a comparatively great thermal expansion factor and manufacture-related deviations in measurements, which is why in particular large sized filter elements must be insertable into the receiving housings with a corresponding play. In order to achieve a sufficient sealing between the filter frame and the receiving housing under the temperature conditions to be expected, the sealing strips has to be formed accordingly. For reasons of manufacturing techniques, the filter elements should be provided with integral sealing lips projecting from their filter element frame, whereby the sealing lips should have thickness so as to always abut the receiving housing. To maintain the elasticity of the sealing lips when the filter element is inserted into the receiving housing, they must not be bent beyond their yielding point in the bending section. With sealing lips of plastic material this means that the sealing lips have to be relatively thin. Yet, this is disadvantageous in the manufacturing process, since the injection mold cavities used in making the plastic filter element frame create a comparatively strong (flow) resistance in the area of the thin sealing lip. This requires increased material pressures and causes greater loads on the injection mold and the molded part, in particular when viscose materials such as polypropylene are used, which has proved advantageous as a material for filter devices of the type referred to above.
It is the object of the invention to provide a filter device that can be produced at low cost and which provides for a reliable sealing of the filter element frame in the housing receiving the same.